According to a recent attempt, the laser dicing technology as disclosed in patent documents 1 through 4 is used to form a modifying region (modifying layer) in a wafer (semiconductor wafer) for fabricating a semiconductor substrate. The modifying region is used as a cutting origin for tearing to cut and separate a wafer into chips (semiconductor chips).
For example, patent document 1 proposes the following technology. A laser beam is radiated to a focusing point in a wafer-shaped process object. In the process object, the multiphoton absorption forms a modifying region (i.e., a modifying region containing a crack region, a modifying region containing a melt treatment region, and a modifying region containing a region with a modified refractive index). The modifying region is used to form a region as a cutting origin along an estimated cut line of the process object for a specified distance inside from a laser beam incident surface of the process object. The process object is cut by tearing based on the region as an origin.
Patent document 1 discloses radiation of laser beams from multiple laser light sources in different directions to a focusing point in a process object (see claim 7 and FIG. 17 in patent document 1). The use of multiple laser light sources can provide a laser beam's focusing point with an electric field strength large enough to generate multiphoton absorption. A continuous wave laser beam provides a smaller instantaneous power than a pulse laser beam, but nevertheless can form a modifying region.
Patent document 1 further discloses provision of a light source unit in which multiple laser light sources are arranged in an array along an estimated cut line (see claim 10 and FIG. 18 in patent document 1). This technology claims to simultaneously form multiple focusing points along the estimated cut line and improve a processing speed.
However, the technology according to patent document 1 forms only one layer of modifying region along a depth direction from the wafer surface. When a wafer is thick, it is technologically difficult to accurately cut and separate the wafer along an estimated cut line. When the technology of patent document 1 radiates laser beams from multiple laser light sources in different directions to the focusing point in a process object, only one layer of modifying region is formed along the depth direction from the wafer surface. The technology shows decreased processing efficiency and low throughput (productivity per unit time) and is unsuited for mass production.
As mentioned above, the technology of patent document 1 radiates laser beams from multiple laser light sources in different directions to the focusing point in a process object. When a laser beam is radiated from a wafer surface, the laser beam is slantwise incident on the wafer surface. A semiconductor device may be damaged due to radiation of a laser beam to a region for forming the semiconductor device on the wafer surface. In consideration for this, an estimated cut line needs to be wide enough to radiate the laser beam. Widening the estimated cut line decreases the number of chips that can be cut from one wafer. The chip yield is limited to increase manufacturing costs for chips.
Patent document 2 proposes the technology to radiate a laser beam to a process object at the laser beam's focusing point in the process object. A modifying region is formed in the process object along an estimated cut line of the process object. Further, the laser beam is radiated to the process object by changing the position of the laser beam's focusing point along the incident direction toward the process object to form multiple modifying regions along the incident direction.
The technology of patent document 2 forms multiple modifying regions along the incident direction to increase origins to cut the process object. Even a thick process object can be cut.
The technology according to patent document 3 proposes a laser processing apparatus to form a modifying region using multiphoton absorption in a wafer-shaped process object. The laser processing apparatus is provided with a condenser lens and moving means. The condenser lens condenses first and second laser beams with different wavelengths into the inside of the process object. The condenser lens generates multiphoton absorption based on the first laser beam's focusing point position and the second laser beam's focusing point position. The moving means relatively moves the focusing points of the first and second laser beams along an estimated cut line of the process object.
Since the technology of patent document 3 uses the first and second laser beams with different wavelengths, a chromatic aberration or the like causes the laser beams to condense into positions with different depths from a process object surface toward the condenser lens. The focusing point of each laser beam is relatively moved along the estimated cut line. A single scan along the estimated cut line can form two modifying regions corresponding to the first and second laser beams. According to the disclosure of patent document 3, one condenser lens condenses three or more laser beams with different wavelengths and radiates the laser beams to a process object. A single scan along the estimated cut line can form three or more modifying regions.
The technology of patent document 2 or 3 forms multiple modifying regions in the depth direction from the wafer surface. Even a thick wafer increases locations as origins to cut the wafer. It is possible to accurately cut to separate a wafer along the estimated cut line.
However, the technology of patent document 2 stepwise changes a position of the laser beam's focusing point in the laser beam's incident direction to the wafer. The processing efficiency is degraded because one modifying region is formed at a time with an interval in the depth direction from the wafer surface. Forming multiple modifying regions is time-consuming. The technology shows low throughput (productivity per unit time) and is unsuited for mass production.
By contrast, the technology of patent document 3 radiates multiple laser beams with different wavelengths to a wafer to simultaneously form multiple modifying regions with different depths corresponding to the laser beams. Compared to the technology of patent document 2, the technology of patent document 3 increases the processing efficiency and can form multiple modifying regions in a short period of time. However, the technology of patent document 3 uses a single laser light source to radiate a laser beam with one type of wavelength. An individual laser light source needs to be provided for each laser beam with a different wavelength, increasing the number of laser light sources. Accordingly, the laser processing apparatus becomes large and increases an installation space. In addition, the laser processing apparatus becomes complicated to increase the number of parts and therefore manufacturing costs.
The technology of patent document 2 uses the laser beam with one type of wavelength to form a modifying region. When a wafer is very thick, it is difficult to reliably form normal modifying regions at a shallow portion and a deep portion from the wafer surface, i.e., the incidence plane for the laser beam.
For example, let us suppose that the laser beam wavelength is set so as to reliably form a normal modifying region at a shallow portion from the wafer surface. In this case, it is difficult to reliably form a normal modifying region at a deep portion from the wafer surface. Reversely, let us suppose that the laser beam wavelength is set so as to reliably form a normal modifying region at a deep portion from the wafer surface. In this case, it is difficult to reliably form a normal modifying region at a shallow portion from the wafer surface.
Recently, the semiconductor substrate multi-layering technology has advanced. An attempt is made to use the laser dicing technologies disclosed in patent documents 1 through 3 for a multi-layer wafer and cut to separate it. The semiconductor substrate multi-layering technology covers, for example, the SOI (Silicon On Insulator) technology including the bonding technology and the SIMOX (Separation by IMplanted OXygen) technology, the crystal growth technology for a III-V compound semiconductor layer on a substrate such as sapphire, and the technology of using anodic bonding to bond a silicon substrate and a glass substrate.
Since the technology according to patent document 1 or 2 uses the laser beam with one type of wavelength to form the modifying region, it is difficult to reliably form a normal modifying region in a multi-layer wafer. The reason may be as follows. Layers of a multi-layer have different optical characteristics. Each layer indicates a specific refractive index for laser beams. A laser beam partly reflects on a boundary surface between the layers. The reflected light interferes with the incident light to be canceled. The laser beam energy greatly attenuates at a deep portion from the incidence plane for the laser beam. The deep portion is short of the energy for laser beam L needed to generate multiphoton absorption. The modifying region cannot be formed.
When a wafer is not reliably provided with a normal modifying regions, the wafer unnecessarily cracks during separation by cutting. Accurate separation by cutting is difficult along an estimated cut line, decreasing the yield and the quality of chips that are cut and separated from the wafer.
The technology proposed in patent document 4 provides a laser dicing apparatus that applies a laser beam from a wafer surface and forms a modifying region in the wafer. The apparatus is provided with multiple laser heads and a chuck table. The laser heads radiate the laser beams to the wafer. The chuck table mounts the wafer and moves to the X direction, i.e., a process direction, relatively to the laser heads. The laser heads are configured to be independently movable in Y directions orthogonal to the X direction.
According to the technology of patent document 4, the multiple laser heads can independently move in the Y direction. Multiple lines can be simultaneously processed on a wafer with various process pitches. This may result in an improved processing efficiency.
Patent document 4 discloses that the multiple laser heads are provided so as to be independently movable in a Z direction orthogonal to the X and Y directions. It is possible to specify different focusing points along the Z direction for laser beams radiated from the multiple laser heads. One process stroke can form multiple layers of modifying regions in the wafer, making it possible to easily tear even a thick wafer.
According to the technology of patent document 4, modifying regions may be formed at a shallow portion first and then at a deep portion from the wafer surface as an incidence plane. In this case, the modifying region formed at the shallow portion hinders incidence of a laser beam for forming the modifying region at the deep portion. It is difficult to reliably form a normal modifying region at a deep portion.    Patent document 1: JP-3408805 B    Patent document 2: JP-2002-205180 A    Patent document 3: JP-2004-337903 A    Patent document 4: JP-2004-111946 A